The present invention relates generally to ionomer composite compositions, and more specifically to such compositions useful for many dental applications, ranging from direct restorative materials to fabrication of preformed structure for dental and osseous tissue repair applications.
It has been established that fluoride ions in the oral environment are beneficial to the reduction of recurrent caries, and formation of new caries. It is not currently known why fluoride ions contribute to a decrease in dental caries; theories include its effect on the bacteria, the formation of fluorapatite, and an increase in resistance to caries of both enamel and dentin. The glass filler used in composites and glass ionomers contains sufficient fluoride to see a reduction in caries in filled and surrounding teeth. In addition, the materials can be “recharged” through fluoride toothpaste and optical fluoride treatments.
It has been shown that dental silicate cements containing fluoride are therapeutic in preventing secondary caries and reducing plaque formation. Acrylic denture base material, restorative materials and adhesives have been shown to be sites of bacterial and plaque accumulation, which can be a precursor of irritation to soft tissues and caries attack on remaining natural dentition. The release of fluoride ion from these dental restorative materials generally occurs either by surface release, or by dissolution of the fluorine-containing additives or the dental restorative material itself with consequent migration of fluoride ions into the underlying tooth structure.
Various fluorine-containing additives that have been tried in dental restorations consist of inorganic fluoride salts, organic bases such as amine hydrofluoride, fluorocarbons and fluoride-containing ion-exchange resins. These attempts to find suitable fluorine-containing additives which are both dispersed in dental restorative material and capable of reducing tooth caries through controlled long-term fluoride release have failed. Silicate cements have demonstrated cariostatic release of fluoride. However, the strictly rapid surface release of fluoride from the cement, the dissolution of the cement in oral fluids, and the low tensile strengths of the cements are major disadvantages. Alternatively, the fluoride incorporated into insoluble resin materials has been considered to be virtually incapable of leaking out, and thus to be ineffective as a cariostatic agent Studies by Forsten and Paunio (Scandinavian Journal of Dental Research (1972) 80: 515–519) comparing fluoride release by silicate cements and composite resins have shown that the overall release of fluoride from the two materials was comparable; however, the manner in which the fluoride was released from the composite was not controlled. It was found difficult to obtain controlled, effective cariostatic and plaque-reducing fluoride release from virtually insoluble materials such as acrylic denture base materials, adhesives and composite resins, and the like.
U.S. Pat. No. 4,772,325 issued to Kwan et al. discloses a controlled, slow fluoride releasing additive comprising a Lewis base and a fluoride-containing Lewis acid which is therapeutic in preventing secondary caries and reducing plaque formation. This additive is incorporated into polymeric dental restorative material and is capable of migrating from the interior to the surface of the material without dissolution thereof and with consequent release of fluoride. However, this composition is merely an additive to existing dental restorative materials. Further, it does not increase desirable mechanical properties of the dental restorative material in which it is incorporated; nor does it improve adhesion between polymers.
Current dental composite resins comprise one end of a continuous spectrum of multi-phase dental materials, with glass ionomers at the other end. Hybrid materials such as resin-modified glass ionomers and compomers lie somewhere between. Whereas both composite resins and glass ionomers have been known and used for a relatively long period of time, the hybrid materials such as the resin-modified glass ionomers and compomers are relatively new.
Ceramic filler reinforced polymeric composites are widely used in the area of dental restorative materials. A typical dental composite is composed of a mixture of silicate glass or quartz particles with an acrylic monomer that is polymerized to form a hardened composite material. In current dental composites, the fillers are mostly glasses, occasionally glass-ceramics and quartz (a crystalline form of silica), particulate polymers and glass-polymer composite particulates. Strategies such as the development of smaller filler particles such as microfiller and nanofiller particles; the improvement of glass compositions; and the increase of filler volume fraction through the use of hybrid and heterogeneous filler systems have each been attempted in order to improve dental composites.
There is an increasing need to extend the use of polymeric composites to stress bearing posterior applications due to concerns about the release of mercury from dental amalgam. However, the relatively low toughness, strength, wear resistance and durability of current dental composites have limited their use. It is generally agreed that current polymeric composites cannot be routinely substituted for dental amalgam and achieve the same clinical results. In the posterior dentition, in situations where occlusal stresses are concentrated, the current composites are inappropriate choices (Corbin and Kohn, 1994, JADA 125: 381–388). The current polymer composites are not recommended for large posterior restorations because of the potential for excessive wear, microleakage or fracture (Bayne et al., 1994, JADA 125: 687–701). Composite restorations, in low stress-bearing applications not involving cusps, have average lifetimes of less than 10 years. In comparison, dental amalgam restorations, in high stress-bearing posterior applications with cusp replacement, have lifetimes of 15 years (Corbin and Kohn, 1994).
Many changes have occurred in recent years concerning dental restorative materials. Among these changes is the use of fluoride-releasing glass ionomer materials. Glass ionomer materials are based on the acid-base reaction of an aqueous solution of a polycarboxylic acid with an ion leachable, fluoride-containing glass (Wilson and Kent, 1971, J. Appl. Chem. Biotechnol. 21: 313; Wilson and Kent, 1972, Br. Dent. J. 132: 133). Glass ionomers are noted for their inherent adhesiveness to teeth and their ability to release fluoride to adjacent tooth structure in a sustained fashion to combat secondary caries.
However, it is generally accepted that glass ionomers possess inferior mechanical properties, including extreme brittleness and low strength (e.g., flexural strength of 10–20 MPa--McLean, 1990, J. Am. Dent. Assoc. 120: 43). This mechanical inferiority has severely limited their use. The reinforcement of glass ionomers by disperse phase corundum (Prosser et al., 1986, J. Dent. Res. 65: 146), alumina fibers and other fibers (Sced and Wilson, Br. Pat. Application GB 2,028,855A, 1978), and metal powders (McLean and Gasser, U.S. Pat. No. 4,527,979, 1985) resulted in only incremental improvement in mechanical properties. Flexural strength values of reinforced glass ionomers have rarely exceeded 50 MPa (Wilson and McLean, Glass-Ionomer Cement, 1988; McLean, J. Am. Dent. Assoc., 1990, 120: 43).
Resin-modified glass ionomers (Mitra, EP Application 323,120, 1988; U.S. Pat. No. 5,154,762, 1992), where compatible resins (e.g., 2-hydroxyethyl methacrylate, or HEMA) are used with the polyacids, are only slightly stronger than glass ionomers (e.g., flexural strength of 60 MPa—Poolthong et al., 1994, Dent. Mater, J. 13: 220; Hickel, 1996, Acad. Dent. Mater. Trans. 9: 105). It has been generally accepted that the most intractable problem is likely to be lack of strength and toughness (Wilson, A. D. and J. W. McLean, Glass Ionomer Cement, Quintessence Publ. Co. (1988)). For a further discussion of glass ionomers as used in the dental market for direct restorative materials, eg. as cure-in-mouth cements, see, for example, Wilson and McLean, (1988); Katsuyama, S., T. Ishikawa, et al., Eds., Glass Ionomer Dental Cement—The Materials and Their Clinical Use, Ishiyaka EuroAmercia Inc. (1993); and O'Brien, W., Dental Materials and Their Selection, Chicago, Quintessence Publ. Co. (1997).
Compomers have been developed recently. See, for example, U.S. Pat. No. 4,816,495 issued to Blackwell et al.; U.S. Pat. No. 5,367,002 issued to Huang et al.; and Blackwell et al., Acad. Dent. Mater. Trans., 1996, 9: 77. Compomers are basically hybrid, glass ionomer-composites modified in their resin phase by a carboxylic acid monomer and in their filler phase by the inclusion of an acid-reactive, ion-leachable glass. The name compomer is derived by combining the two words composite and ionomer, and is intended to suggest a combination of composite and glass-ionomer technology. The liquid part of a compomer is a mixture of a dental resin monomer (such as UDMA, a urethane dimethacrylate) and a carboxylic acid monomer (e.g., TCB, the reaction product of butane tetracarboxylic acid with HEMA), with the resin being the major phase and TCB the minor phase. The filler part of a compomer is a mixture of dental silicate glass and reactive fluorosilicate glass particles, with the reactive glass being the minor phase.
In contrast to glass ionomers, compomers do not generally contain significant amounts of water. The sole initial curing reaction is radical induced polymerization of the acrylic resin monomer matrix. An acid-base reaction takes place between TCB and the ion leachable fluorosilicate glass only after water infuses the cured composite via exposure to oral fluids, which also causes the filling to release fluoride ions. Flexural strength values of 90–125 MPa have been reported for compomers (Hickel, 1996, Acad. Dent. Mater. Trans. 9: 105). However, these strength values are still inferior to those of current dental amalgam (100–150 MPa) and composite resins (100–145 MPa) (Hickel, 1996). Therefore, compomers are currently not recommended for use in large, stress-bearing posterior applications.
It has been generally recognized that glasses and glass-ceramics are among the weakest and most brittle materials to use as reinforcement fillers. Glass filler particles are sensitive to surface flaws produced during mixing, handling and wear. A crack in the resin matrix can easily cut through the reinforcing glass particles.
U.S. Pat. No. 5,861,445 issued to Xu et al. further defined geometrical shapes of the filler particles as cause for failure. The previously known glass fillers were either spherical or of irregular shapes, with length-to-diameter ratio only slightly larger than one. Xu et al., disclosed that this had at least two major short-comings. First, rounded filler particles at occlusal surfaces are susceptible to facile dislodgement from the resin matrix during wear with foods bolus, resulting in high wear rates. Second, if a crack is initiated in the composite, it can easily propagate around the filler particles, hence causing the reinforcing effect of the filler particles to be lost.
As an answer to those shortcomings, Xu et al. proposed the use of ceramic filler particles and whiskers and/or chopped fibers to reinforce polymeric dental composites so that there are substantially improved mechanical properties and enhanced clinical longevity compared to conventional (or currently used) materials. The elongated whiskers and chopped fibers were said to have high length-to-diameter ratio values to effectively bridge and resist micro-cracks. In addition, it was asserted that the whiskers were less likely to be dislodged out of the matrix during wear.
In the area of conventional preformed structure for dental and osseous tissue repair applications, there are several shortcomings inherent therein. For example, current plastic teeth offer low abrasion resistance, low crazing resistance, and low heat-distortion temperatures when compared to porcelain teeth. See, for example, Craig, R., Ed., Restorative Dental Materials, St. Louis, Mosby (1997). If these problems could be overcome, plastic denture teeth are already superior to porcelain teeth in toughness, natural feel, and ease of grinding and polishing (Craig 1997).
Thus, it is an object of the present invention to provide an ionomeric composite composition for dental application(s) which advantageously improves the wear properties of the composition in the dental application. It is a further object of the present invention to provide such a composition having a highly crosslinked structure, thereby advantageously improving the strength and crazing resistance of the composition in the dental application. Still further, it is an object of the present invention to provide such a composition which utilizes the unique properties of glass ionomers by advantageously adapting them for use in preformed denture teeth and other preformed structure. It is yet another object of the present invention to provide an ionomeric composite advantageously utilizing a copolymer as one component thereof. Yet still further, it is an object of the present invention to provide such a composition which may advantageously be varied to suit a wide variety of dental applications, ranging from direct restorative materials, to intermediary dental materials such as liners, bases, and luting cement, to preformed structure for dental and osseous tissue repair applications. Yet further, it is an object of the present invention to provide such a composition which may advantageously be used for implants and/or tissue scaffolding (growing natural tissue within/on a porous synthetic material). It is still further an object of the present invention to provide such a composition which advantageously provides continuous fluoride release.